Hickok Tube Tester Questions

I have a Hickok 600 in good condition and recently calibrated.

I've often read that it should not be left on for more than an hour and that heat buildup is one of the causes of variable test results. So, is it advisable to take the machinery out of the case and use it that way to keep the temperature stable and below overheating levels?

Also, I frequently see 12ax7s advertised as testing around 2000 micromhos where 1250 is the nominal new reading given on the 600's roll chart. I think that these ultra high readings are the result of incorrectly set English settings (also called Shunt). The roll chart gives an English setting of 58 but this setting is supposed to be used for the good/bad test only. When testing for micromhos the red dot setting of around 72 is the correct English setting.

I know that other testers use different scales but if the reading is given in micromhos or Gm the same tube should give similar, not exact, results in different testers. Do I have this right?
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You could probably put it in a ventilated cabinet but the big question is,is the transformer rated for continuous duty?
I have the Military Cardmatic(USM-118B)version of the Hickok testers and the scale on my meter is from 0 to 100. I have a test condition book that tells me the micromhos for a 12AX7 on this tester at full scale (100) would be 1600. My 12AX7s all test around 75 to 78 (math translation 75% to 78% of 1600 - 1200 to 1250 micromhos). I am comfortable with these numbers but it took me some time to realize many other testers and tester types will have very different numbers as being "good" mutual conductance.